CompTIA is good, but you're right, they'll probably struggle with the human side of things for a bit. If they can stick out help desk work they may be able to transition into system administrator work, which is less customer facing.

From there, DevOps engineering is, I've found, a good place to aim long term for those with communication difficulties. You're usually pretty insulated from clients, at the expense of not being able to work on the "flashy" stuff that directly generates profit for the company.

It's definitely work that emphasizes a steady, methodical approach to things, and really knowing your tools well. Have them take a look at https://roadmap.sh/devops and see if they like what it looks like.